also make plotfile writes use nonblocking APIs

git-svn-id: https://svn.mcs.anl.gov/repos/parallel-netcdf/trunk@1617 c368ceb8-fd28-0410-a457-e46ee1d7cd4a
variable i may be declared but not used

git-svn-id: https://svn.mcs.anl.gov/repos/parallel-netcdf/trunk@1618 c368ceb8-fd28-0410-a457-e46ee1d7cd4a